Caffeine has a stronger disruptive effect on daytime, catch-up sleep after a night of sleep deprivation than it does on a normal night’s sleep, a new study shows.

“Recent studies have suggested that caffeine is a measure of choice to counteract the effects of sleep deprivation on alertness and performance,” Dr. Julie Carrier of the Hopital du Sacre-Coeur de Montreal in Quebec and colleagues note.

“Our results suggest that this recommendation should be made with caution since using caffeine before sleeping at an abnormal circadian phase, such as during night work, might have more adverse consequences on sleep quality than before sleeping at a normal circadian time.”

Consuming caffeine at night is known to make it more difficult to fall asleep and to worsen sleep quality, Carrier and her team note. They hypothesized that caffeine would have an even stronger effect when people were trying to recover from sleep deprivation by snoozing during the day.

To investigate, they had 34 people consume caffeine or placebo before going to sleep at their normal bedtime. A month later, the same individuals were then deprived of sleep for an entire night, after which they were instructed to fall asleep an hour after their normal wake-up time, again after being given placebo or caffeine. People received 100 mg of caffeine or placebo before their scheduled morning or evening bedtime, and then another 100 mg (or placebo) one hour before.

All study participants who took caffeine took longer to fall asleep and spent more time in lighter stage 1 sleep and less time in deeper stage 2 and slow-wave sleep, the researchers found. But people given caffeine before daytime sleep took even longer to drop off, and also slept for a shorter total time and had less REM sleep. Caffeine didn’t affect nighttime sleep duration or REM.

Carrier and her colleagues suggest that the greater daytime effects of caffeine were due to the fact that people were trying to fall asleep at a time when their body clock was telling them to stay awake, even though they were sleep-deprived. Slow-wave sleep is known to last longer during daytime, catch-up sleep, they add, which normally helps override this circadian signal. Given that caffeine reduces slow-wave sleep, it appears to allow the wake-up call to remain strong.

SOURCE: Neuropsychopharmacology, August 23, 2006.